devex: pin Rust toolchain 1.94.1 with rustfmt, clippy, and wasm target (closes #159)

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Description

Pins the repository's Rust compiler toolchain using rust-toolchain.toml to prevent sudden CI failures caused by upstream clippy lints or toolchain changes:

• Channel pinned to 1.94.1.
• Components: rustfmt, clippy.
• Targets: wasm32v1-none (required for compiling the deprader_router Soroban contract).
• Added instructions to CONTRIBUTING.md documenting how maintainers and contributors can bump the pinned version.

Closes #159

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Tasks Completed

• Created rust-toolchain.toml pinning 1.94.1 with rustfmt, clippy, and wasm32v1-none.
• Documented the toolchain bump procedure in CONTRIBUTING.md.
• Verified that cargo fmt --all -- --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo test --workspace --locked, and cargo build -p deprader-router --target wasm32v1-none --release --locked all pass under the pinned toolchain.
• Added progress entry in PROGRESS.md.

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Acceptance Criteria

• cargo fmt, cargo clippy -- -D warnings and cargo test --workspace pass
• PROGRESS.md entry added, and the PR description includes Closes #159

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Stellar Address (Stellar Wave Program)

GDLCJGQLXZGDAAT5QGRF6FM2MBEER3FPYGCSVPXYL47G2ABIOX2M2S6K